1. Field of the Invention
The invention relates in general to a signal coupling method and circuit, and more particularly to a signal coupling method and circuit using a common reference input.
2. Description of the Related Art
When an integrated circuit (IC) device processes a signal, owing that the voltage required inside the device is different from the voltage used outside the device, the ac component of the signal is usually coupled into the device in a capacitor coupling way and then the required voltage level of the signal is given according to various application conditions. If a reference signal of the signal can also be coupled into the device when the signal is being coupled into the device, the device can have better noise-resistance effect. Referring to FIG. 1, a schematic diagram of a conventional signal coupling circuit is shown. The IC device 100 includes a processing circuit 115. The processing circuit 115 has a positive input terminal for receiving input signals and a negative input terminal for receiving reference signals of the input signals. The input signals sig_A, sig_B, and sig_C are respectively coupled to the IC device 100 via the capacitors 121, 122, and 123. The IC device 100 further includes a multiplexer (MUX) 130 connected between the capacitors 121, 122 and 123, and the processing circuit 115 for selectively outputting one of the three signals sig_A, sig_B, and sig_C to the processing circuit 115. Similarly, the reference signals ref_A, ref_B, and ref_C corresponding to the signals sig_A, sig_B, and sig_C are respectively coupled to the IC device via the capacitors 124, 125, and 126, and a multiplexer 131 connected between the capacitors 124, 125 and 126, and the processing circuit 115 is used for selectively outputting one of the three reference signals ref_A, ref_B, and ref_C to the processing circuit 115.
For example, when the processing circuit 115 is to process the signal sig_B, the multiplexer 130 selects to output the signal sig_B to the positive input terminal of the processing circuit 115 in the meanwhile the multiplexer 131 selects to output the reference signal ref_B of the signal sig_B to the negative terminal of the processing circuit 115. However, when the number of the input signals is increased, the number of the corresponding reference signals will be increased accordingly. As a result, the IC device 100 needs to have more pins for signals and the circuit board needs more coupling capacitors, thereby increasing circuit cost.